BRUSSELS, Aug 6 (Reuters) - Russia has amassed around 20,000 troops on Ukraine's eastern border and could use the excuse of an humanitarian or peacekeeping mission to send them into Ukraine, NATO said in a statement on Wednesday.

"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground - and that is of great concern. Russia has amassed around 20,000 combat-ready troops on Ukraine's eastern border," NATO spokeswoman Oana Lungescu said in a statement.

"The latest Russian military build-up further escalates the situation and undermines efforts aimed at finding a diplomatic solution to the crisis. This is a dangerous situation."

Polish Prime Minister Donald Tusk said on Wednesday the threat of a direct intervention by Russia's military in Ukraine has risen over the last couple of days.

NATO's statement said it was concerned Moscow could use "the pretext of a humanitarian or peacekeeping mission as an excuse to send troops into Eastern Ukraine." (Reporting by Barbara Lewis and Tom Koerkemeier; editing by Foo Yun Chee)

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